Motor innervation
ANSC (FSTC) 607
Physiology and Biochemistry of Muscle as a Food
MOTOR INNERVATION AND MUSCLE CONTRACTION
I. Motor innervation of muscle
A. Motor neuron
1. Branched (can innervate many myofibers) à terminal axons.
2. Absolute terminal innervation ratio: one myofiber innervated by one terminal axon.
B. The motor unit
1. Each motor neuron innervates several muscle fibers within a muscle.
2. Size of motor unit varies with muscle and fineness of movement.
a. There are 100 to 200 myofibers per motor neuron in larger muscles.
-- rat soleus, 200 fibers/neuron; rat gastrocnemius, 1,000 fibers/neuron
b. There are fewer in muscles such as ocular muscles.
C. The neuromuscular junction
1. Terminal axon
a. Vesicles (contain acetylcholine)
b. Presynaptic membrane
c. Synaptic cleft
2. Myofiber
a. Postsynaptic membrane – sarcolemma
b. Synaptic clefts – increase surface area
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Motor innervation
D. Transmission of impulse across the synaptic cleft – synaptic delay of the action potential
1. Acetylcholine
a. End-plate potentials
b. Quantal nature of transmitter release – each vesicle contains 103 to 104 molecules
of acetylcholine
2. Acetylcholinesterase
a. In synaptic cleft degrades acetylcholine
b. Stops transmission signal, contraction
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Motor innervation
II. Neurotransmitter release
A. Neurotransmitter released from the
presynaptic vesicles
B. Stenin (actin-like) is associated with
vesicles and neurin (myosin-like) is
associated with the presynaptic
membrane.
C. Calcium influx causes stenin to fuse
with neurin.
D. This activates neurostenin ATPase;
vesicles expel 1,000 to 10,000
molecules of acetycholine into cleft.
IV. Depolarization of the sarcolemma
A. Depolarization of postsynaptic membrane
1. Causes local depolarizaton (miniature end-plate potentials).
2. Miniature end-plate potentials are caused by the quantal release of acetylcholine.
B. Depolarization of sarcolemma
1. Depolarization = summation of miniature end-plate potentials leads to action potential.
2. Action potential spreads across sarcolemma.
3. Action potential reaches interior via the t-tubules.
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Motor innervation
VI. Generation of the resting membrane potential
A. Resting state (steady state)
requirements
1. Equimolarity
2. Electrical neutrality
3. Zero electrochemical gradient
B. Basis for the resting membrane
potential
1. Ions responsible are primarily
Na+ and K+.
2. Factors influencing magnitude of
the action potential are primarily
the concentrations of Na+ and K+.
C. Calculation of the resting membrane potential
1. Nernst equation: E = (-RT)/zF)ln[Ki]/Ko]
Where E = potential difference across the membrane (usually in mV)
R = gas constant
T = absolute temperature
F = Faraday's constant (# charges per mole ion)
z = valence of ion
2. Modified Nernst equation
R and F = constants
T (20°C) = 293 absolute
z for K+ = 1
Convert ln to log10, so that: E (in mV) = -58 log10[Ki]/[Ko]
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Motor innervation
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Motor innervation
VII. Initiation of contraction
A. Action potential causes release of Ca++ from the sarcoplasmic reticulum.
1. Release of Ca++ occurs in area of triad (at A band-to-I band interface in mammals).
2. The dihydropyridine receptor (embedded in the T-tubule) is altered by the incoming
action potential.
2. This causes the ryanodine receptor (underlying the T-tubule) to open, allowing calcium
release from the sarcoplasmic reticulum.
3. Ca++ follows its electrochemical gradient into the sarcoplasm.
4. The sarcoplasmic concentration of Ca++ increases from 10 nM to 10 µM (1,000-fold
increase).
B. Ca++ binds to troponin C, which initiates contraction.
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Motor innervation
VIII. Return to resting state
A. Acetylcholine levels are reduced.
1. Acetylcholine is repackaged into vesicles.
2. Acetylcholine also can be degraded to acetate and choline by acetylcholinesterase.
B. Sarcoplasmic Ca++ resequestered in sarcoplasmic reticulum by:
1. Ca++-Mg++-ATPase (active pumping)
2. Calsequestrin
a. Binds 43 Ca++/mol of protein.
b. Helps to concentrate Ca++.
3. High-affinity Ca++-binding protein also helps to sequester Ca++..
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